In general, reinforcement of fragile thermoplastic polymers by the incorporation of a rubber, brings about in them considerable increase of their firmness, and thus making them impact-resistant material. In this sense, it is well-known that the polystyrene (PS), and the poly(styrene-co-acrylonitrile (PSAN) are thermoplastic polymers with a low firmness. However, when they are reinforced with an elastomer it is obtained the high-impact polystyrene (HIPS) and poly(acrylonitrile-bultadiene-styrene) (ABS), impact-resistant thermoplastic materials, which are formed by a rigid PS and PSAN matrix, respectively, with rubber particles dispersed in such matrixes. Likewise, final properties of these materials depend, among several other parameters, upon the size and type of morphology of present particles. Therefore, a deviation from the HIPS and BS properties and features may be achieved to the extent in which rubber particles are able to present different morphologies.
In this way, there are several and well-known strategies in the state of the art in order to produce changes in size and type of particles in a HIPS and ABS. The most important factor is the type of rubber used. Thus, styrene/butadiene copolymers represent an excellent alternative to obtain HIPS and ABS since through such copolymers and through variation in composition of polystyrene in the copolymer, particles with different morphologies may be obtained (Angew. Makromol, Chem. 90 (1980) 95, and Angew. Makromol. Chem 175 (1977) 58). In addition, by using this type of elastomers, it is possible to combine in HIPSs and ABSs, good mechanical and optical properties. Due to the above, several research have been carried out prior to this one, focusing their efforts in obtaining HIPS and ABS, and using to that end an extensive variety of styrene/butadiene copolymers in order to obtain materials with a suitable balance in their mechanical and optical properties and mainly due to the different morphologies of particles present in HIPS and ABS.
For example, U.S. Pat. No. 4,838,418 details the obtainment of HIPS mainly with capsular morphology through polymerization of styrene in presence of a styrene/butadiene copolymer with a 40/60 ratio and a chain transference agent as regulator of the of the polystyrene matrix molecular weight. On the other hand, U.S. Pat. No. 4,771,107 outlines the use of styrene/butadiene copolymers with a high-styrene content to produce ABS with good transparency. On its part, U.S. Pat. No. 5,223,577 by means of using styrene/butadiene copolymers, and from polymerization of styrene and acrylonitrile in presence of such copolymers, details how to obtain ABS with good optical and mechanical properties.
More recently, U.S. Pat. No. 5,756,579 details the use of styrene/butadiene copolymers with a low-styrene content to produce ABS with excellent balance between the physical and mechanical properties. U.S. Pat. No. 4,990,236 reports the synthesis of impact-resistant material through the in situ formation of an implanted copolymer by using, to that end, different SBR-, SBS-, BSB- and/or SBSB-type styrene/butadiene copolymers with different compositions, and styrene by means of a polymerization process in solution.
On the other hand, U.S. Pat. Nos. 5,428,104, and 5,591,195 outline the HIPS synthesis by using block copolymers of styrene/butadiene with a 30/70 ratio, where the particles obtained present capsule and hank-type morphologies, while the materials have a good firmness together with a high brightness. The reason for which different morphologies are obtained by using the same copolymer is due to the use of different concentrations of a chain transference agent during the synthesis process.
On the other hand, U.S. Pat. No. 5,473,014 details the production of HIPS with different morphologies by using mixtures of styrene/butadiene copolymers with different compositions, or else, by means of the joint action of polybutadiene and styrene/butadiene copolymers in different proportions. Therefore, capsule-, cell- and roll-type morphologies are featured and materials obtained display high brightness and great impact resistance, at the same time.
U.S. Pat. Nos. 5,985,997 and 4,524,180 outline the styrene polymerization reaction in presence of polybutadiene and a styrene/butadiene block copolymer with a 40/60 ratio. The obtained HIPS feature a bimodal distribution of particles with capsule-type morphologies, with a particle size of 0.2 to 0.6 μm, and particles with salami-type morphologies with a size of 1.2 to 8 μm. The presence of such particle types and sizes give the material, at the same time, good mechanical and optical properties.
From what has been outlined above, it can be mentioned that although all the afore-cited patents detail how to obtain the HIPS featuring different morphologies, size, and distribution of particles, they are obtained by using chain transference agents, which regulate the implanting reaction of styrene on the rubber; or else, by jointly using polybutadiene and/or styrene/butadiene copolymers in different compositions.